1. Field of the Invention
This invention relates to thyratrons and more specifically to thyratrons of the kind having a so-called primary grid in addition to the normally provided control grid between cathode and anode, said primary grid being located between said control grid and the cathode. A thyratron is a gas-filled tube in which one or more control electrodes initiate, but do not limit, the anode current, except under certain operating conditions.
2. Description of Prior Art
A typical thryratron of the kind referred to as at present known is illustrated schematically in FIG. 1.
Referring to FIG. 1 the thyratron comprises a cathode 1 and an anode 2 having therebetween a control grid 3 and a primary grid 4. As will be seen, the primary grid 4 is located between the control grid 3 and the cathode 1 and has two arcuate coupling slots 4a and 4b therein.
A primary discharge represented at 5 is maintained between the primary grid 4 and the cathode 1 by means of a positive potential applied to the primary grid 4. The gap 6 between the control grid 3 and the primary grid 4 constitutes the region in which the main discharge occurs when the thryatron is triggered into its conductive state by applying a sufficiently high positive potential to the control grid 3. Once triggered, the thyratron remains in a conductive state until the anode potential is reduced to zero or made negative. Normally, in order to ensure that the thyratron is held in a non-conductive state prior to triggering, the control grid 3 is maintained at a negative potential, preventing the main discharge from occurring in the gap 6.
The purpose of providing the primary grid 4, and thus the ability to maintain a primary discharge between it and the cathode 1, is to reduce the time required for the anode current to reach its maximum value by decreasing the time required to produce the main discharge. The maintenance of the primary discharge also tends to reduce pulse to pulse jitter which is partly caused by vibration in the threshold voltage required to trigger the thyratron on different occasions.
Increasing the current in the primary discharge will shorten the anode current rise time but with the attendant disadvantage that the discharge may penetrate through the primary grid 4 into the gap 6 where the main discharge occurs, thus increasing the probability of producing the main discharge prematurely. This effect can be combated by increasing the negative bias voltage on the control grid 3 but above a certain limit this ceases to be effective.